Concrete in Australia Vol 39 No 3 55
8.0 MONITORING OF GEOPOLYMER
CONCRETE RETAINING WALLS
In order to obtain a greater understanding of the practical
potential of geopolymer concrete, in 2009 VicRoads
undertook a small number of trials including the in-situ
construction of two landscape retaining walls at a bridge
over the Yarra River (Figure 17) (10, 11). Construction of
the in-situ geopolymer concrete landscape retaining walls was
undertaken utilising conventional techniques for formwork
construction, concrete placement by pumping, compaction
with a poker vibrator, and finishing and curing with
polyethylene plastic.
In order to monitor the long-term performance of the
geopolymer concrete and enable monitoring of the corrosion
state of the reinforcing steel, three MnO2 half-cell reference
electrodes were installed at the centre of each of the in-situ walls
adjacent to the steel reinforcement at three different levels along
the height of the wall (Figure 17).
Initial measurement of the potentials of the steel
reinforcement against the reference electrodes commenced a few
weeks after construction in 2009, and subsequently monitored
on a regular basis. e initial half-cell potentials readings after
the hardening of the geopolymer concrete were very negative,
namely in the order of -600 to -800 mV for upstream wall and
about -1000 mV for the downstream wall, reflecting the initial
quality of the two walls. e half-cell potential of the steel in
concrete, however, appeared to be stabilising over the following
six months after construction with the potentials having shifted
to more positive values by about 200 mV, as shown by the
results of the monitoring system incorporated in the walls
(Figure 18).
Further measurements on the embedded reference electrodes
in 2011/2012 showed that the half-cell potentials of both wing
walls have become more positive since the 2010 measurements
(Figure 18), and are stabilising between -350 mV and -250 mV
(CSE) which based on conventional criteria, it is unlikely
that corrosion of the steel is taking place. ese values may
become even more positive, at least in some areas of wing walls,
indicating that the corrosion risk is not significant at present.
is is in agreement with results of very low penetrability to
chloride ions (ASTM C1202) and very low chloride diffusion
coefficient determined using the NT Build 443 test method. It
should be noted however, that the VPV (volume of permeable
voids) values to AS 1012.21 did not comply with the criterion
of a maximum value of 16% for structural concrete of
VR400/40 grade as set out in Section 610 (5). Nevertheless, it is
argued that the higher VPV is not due to larger interconnected
pore volume, but due to additional loss of water from the gel-
like materials included in the geopolymer. It is likely that an
excess amount of sodium silicate (which releases water as part of
the chemical reaction) was used in the geopolymer formulation,
which was not fully assimilated into the geopolymer binder and
caused the high VPV. It is considered that further refinement of
the geopolymer concrete mix design with the use of compatible
water reducers and superplasticisers
to reduce the amount of water in
the mix will significantly reduce the
VPV of the geopolymer concrete.
9.0 CONCLUSION
Corrosion monitoring sensors
may be cast into new structures or
installed in existing structures alone
or as part of repairs, to provide
the asset manager with real time
information as to the current state
and performance of the structure
or remedial works. Monitoring
sensors can provide early detection
of initiation and/or propagation of
corrosion and therefore facilitate
early diagnostic assessment and
Figure 17. Finished painted geopolymer concrete wall and installed reference electrodes.
Figure 18. Monitoring of reinforcement potentials -- Bridge over Yarra River west retaining walls.